1. Field of the Invention
The present invention relates to a pseudo-random sequence generator arrangement, including at least two basic shift registers which are arranged as multipliers, the shifts being controlled by pulses from a clock, the initial contents of the registers being preset by an internal key, and which are fed back to each other via a modifying member incorporated in the feedback loop
2. Description of the Related Art
Such an arrangement is disclosed in the European Patent Application EP-A-0,119,972. The arrangement described in said document comprises a plurality of pairs of registers whose outputs are combined in a logic circuit. In each pair, the registers are fed back to each other via a modifying member inserted in the loop, namely a change-over switch which has for its effect that either the main register is fed back to itself, or it is fed back via the second register. In addition, the second register is "disturbed" because of the fact that the clock pulse is applied thereto in bursts as a function of a bit count at the output.
Major problems, as regards the pseudo-random generators, are the following problems:
the chance of detection of its operating mode must be reduced. This means that it must be difficult to discover how the pseudo-random generator operates by observation of its output signals. For a linear generator, of the order n, it is sufficient to observe 2n-1 consecutive elements of a sequence to characterize the generator completely. For non-linear generators it is necessary to observe many more elements;
one can try to detect the structure of the pseudo-random generator by a simulated data processing in real time of the multiplying polynomials, their feedback, and their interconnection, investigating different structures in turns. This simulation must be rendered sufficiently long to ensure that it becomes impossible to realize in actual practice. Nevertheless, the hardware devices must not be too complicated to implement. Also in that case a non-linear generator is more difficult to simulate;
the supplied sequences must have an adequately random character, that is to say they must obey the same laws as a real random sequence (wherein, for example, the statistic proportion of 1 bit to 0 bits is 50%). Moreover, the duration of the cycle, that is to say the cycle before a new identical sequence is found (assuming that the contents are not reinitialized) must be adequately long. With linear generators, the duration can be at its maximum and be constant and the sequences are statistically random. In contrast thereto, with non-linear generators, the duration of a cycle is variable and there is a risk of deterioration, resulting in the appearance of cycles which are too short and/or statistically not random.